Fused coil having positive break



Oct. 8, 1968 w. c. ANDERSON FUSED COIL HAVING POSITIVE BREAK 2 Sheets-Sheet 1 Filed May 13, 1966 FIELE /I II I I I I Oct. 8, 1968 w. c. ANDERSON FUSED COIL HAVING POSITIVE BREAK 2 Sheets-Sheet 2 Filed May 13, 1966 Q C O Patented Oct. 8, 1968 3,405,317 FUSED COIL HAVING POSITIVE BREAK William C. Anderson, Winfield, Ill., assignor to Jefferson Electric Company, a corporation of Delaware Filed May 13, 1966, Ser. No. 549,917 8 Claims. (Cl. 317-15) ABSTRACT OF THE DISCLOSURE An electric coil includes a thermally responsive fuse connected between the finish end and the coil lead. The fuse comprises a strip of metal foil, the metal being a fusible alloy, melting point 125 C. The coil wrapper is lapped over itself to provide a flap, the foil strip being located beneath the flap, together with the finish end and the lead. In a modification, the flap has windows exposing the fuse so that the finish end and the coil lead can extend through the windows. The foil strip may have terminals of tinned copper strips welded thereto.

Inanother modification, the foil strip is connected to the start end, and located between the winding tube and the winding. The winding tube has a window to accommodate the coil lead. This modification can be wound as a part of a multicoil stick on a continuous winding tube having a plurality of foil strips connected end to end by tinned copper strips.

This invention relates to a fused coil, especially suitable for use in potted electrical apparatus such as transformers, reactors, and ballasts for which thermal protection is desired.

Heretofore, thermal protection of such apparatus has been provided by embedding a fuse in the potting material. Such arrangements require a space providing means, such as a cartridge or a container, for the fusible link so that it will drop at the melting temperature.

It is an object of my invention to provide improved thermal protection for potted apparatus which eliminates the space providing means.

Another object is to provide thermal protective means which is a part of the coil itself.

According to my invention, I provide a fuse link in the form of a strip of foil which is embedded in the coil structure.

I have found that where the fuse link is a strip of foil of a fusible alloy, and where it is supported directly on the coil structure in heat transferring relationship, that at a certain temperature, the foil disintegrates. That is, portions of the foil strip simply disappear and the coil can thus be open circuited. The action is more positive than the ordinary fuse in which the break depends on a drop, and the temperature of the break can be more closely controlled because of the more intimate association of the fuse element and the heat generating element, namely, the coil itself.

FIG. 1 is a perspective view of one embodiment of my invention;

FIG. 2 is a view similar to FIG. 1 showing a modification;

FIG. 3 shows a modified fuse means in elevation;

FIG. 4 is a view similar to FIG. 1 but showing a further modification;

FIG. 5 is an elevation of a fuse link assembly suitable for multiwound coils;

FIG. 6 is a fragmentary perspective view of the winding tube used in winding multiwound coils;

FIG. 7 is a perspective view of one of the multiwound coils embodying my invention;

FIG. 8 shows the invention as applied to a ballast; and

FIG. 9 is a fragmentary plan view of FIG. 6.

With reference now to FIG. 1, the coil 10 is a winding suitable for a reactor, transformer, or ballast. It comprises a coil of wire having a start end 11 and a finish end 12, the coil being wound in layers which are separated by paper separators 13. The coil 10 is wound on a cardboard Winding tube 14 of rectangular cross-section. The completed coil also includes a wrapper 15 of paper or other suitable material, the wrapper being lapped over itself by a half turn so that a flap portion 17 can be folded back to expose an underlying portion of the wrapper 15.

A fuse strip 16 is disposed between the overlapped portions of the wrapper 15, and as shown in FIG. 1 is exposed by folding back the fiap 17. The finish end 12 of the coil is secured to one end of the fuse strip 16, as by soldering or spot welding, and a coil lead 18 similarly is connected to the other end of the fuse strip 16. Thus the fuse strip 16 is electrically connected in series with the winding of the coil 10. The connections are indicated by the junctures 19. After the junctures 19 have been made, the flap 17 is folded over the fuse strip 16 and glued or taped in place.

The fuse strip 16 consists of a strip of metal foil made from a fusible alloy, referred to herein as a fusible foil strip. In a preferred embodiment, the strip is exceedingly thin, approximately .003. The width is sufficient so that the total cross section of the fuse strip 16 is sufiicient to carry the current to be drawn by the coil 10. In the FIG. 8 example shown, where the coil 10 is the primary winding of a two 40 watt lamp ballast which would draw somewhat less than one ampere, a quarter inch width is sulficient, giving a total cross sectional area of .00'075 square inch.

The fusible alloy of which the fusible foil strip 16 is composed is any one of a number of well known two, three, and four component mixtures of lead, tin, bismuth, and cadmium, the composition and proportions being selected in accordance with the particular melting point desired. An example of a 135 C. alloy is 32% Pb, 45% Sn, 5% Bi, 18% Cd. In the FIG. 1 arrangement as applied to the FIG. 8 ballast, the circuit opened at a casing temperature of C.

The heat developed by the coil winding 10, while insufiicient to affect the fuse strip 16 in normal operation, will nevertheless cause fusion thereof during certain abnormal conditions which are undesirable. As previously pointed out, the extreme thinness of the fusible foil strip insures a positive break at the fusion temperature.

In the FIG. 2 modification, the outer end of the wrapper 15 is provided with windows 20 and 21 which overlie the opposite ends of the fusible foil strip 16. Thus it is possible to apply the wrapper and tape the coil prior to making the soldered or spot welded junctures 19. Also, in this modification, the start end of the winding is connected to a tinned copper start strip 22, a portion of which is exposed by a notch or window 24 formed in the cardboard tube 14. This permits the coil lead 23 to be welded or soldered to the start strip 22.

In FIG. 3, the fuse means to be embedded in the coil 10 or 10 is a composite element in the form of a thermal protection unit 25. This comprises a fusible foil strip 26 plus two tinned copper strips 27 which are electrically connected thereto, as by spot welding 28. The thermal protective unit 25 is made up in advance, and then is substituted in'either'of the FIG. 1, FIG. 2, or FIG. 4 embodiments for the fusible foil strip 16. This arrangement has the advantage that the spot welds 28 can be effected under controlled conditions so as to avoid a fusing of the fusible foil strip 26, and then after the thermal protective unit is placed on the coil 10, the finish end 12 and the coil lead 18 can be soldered to the opposite ends of the copper strips 27. It is easier to effect a soldered connection to the tinned copper strip than it is to the fusible foil strip itself, and at the same time there is less danger of damage to the fusible foil strip due to conduction of heat from the soldering iron.

FIG. 4 shows a further modification in which the fusible foil strip 30'is disposed around the coil or a portion thereof, rather than being disposed parallel to the coil axis as in FIGS. 1 and 2. Here too, the circumferential fusible foil strip may be located between two overlapping portions of the wrapper, but in the modification shown in FIG. 4, it is disposed between the external surface of the wrapper and a separate cover sheet 31, which extends only partially around the coil. This arrangement has the advantage that the coil 10" may be heat treated after the wrapper 15 has been applied and taped in place. Then after heat treating, the fusible foil strip 30 is applied and is maintained in place by the cover sheet 31-. FIG. 4 shows also an arrangement in which one end of the fuse strip is exposed through a window 32, and the other end extends beyond the edge of the cover sheet 31.

In FIG. 5, the thermal protective units of FIG. 3 are made up in a continuous unit 35 suitabl for multicoil winding, with the fusible foil strip being located at the inside of the coil, rather than at the outer part thereof. For instance, when the coil is a part of a core and coil unit, it is found that the inner surface of the coil runs at somewhat higher temperatures than the outer surface. Thus, by locating the fusible foil strip at the inner surface, it is possible to use a higher melting point alloy, and this in turn permits the use of higher potting temperatures without damage to the fusible foil strip during the potting operation.

In FIG. 5, the unit 35 comprises a plurality of fusible foil strips 36 which are connected end to end by tinned copper strips 37 and secured thereto by spot welds 38 or other suitable means. The multicoil winding operation contemplates ultimate severance of the unit 35 through the copper strips 37, as indicated by the cut lines 40. It also contemplates a soldered connection between the start end 44 of each winding and the unit 35 at the points 39.

In FIG. 6, the unit 35 has been placed over a continuous cardboard winding tube 42, and the soldered connections 39 have been made with the start ends 44. The cardboard tube 42 is provided with a plurality of openings 43 which, as shown in FIG. 9, underlie a portion of each copper strip 37. The start ends are held in place by holding tape 45 and a strain relief loop is preferably provided between the soldered connection 39 and the tape 45 in each start end 44.

According to this method of fabrication, the FIG. 6 assembly is made after the cardboard tube 42 has been placed on the arbor of a multicoil Winding machine. The arbor is then rotated so that a number of coils are wound on the common cardboard tube 42. When the resulting assembly, referred to as a stick, is removed from the arbor, each winding is separate from an adjacent winding, and the stick is cut at points along the cut lines 40 (FIGS. 5 and 9), thus severing the cardboard tube and also the copper strips 37. As a result, each finished coil 49, as shown in FIG. 7, embodies a thermal protective unit like that shown in FIG. 3, which is located between the cardboard tube 42 and the first layer of the winding. The window 43 permits a coil lead 42 to be soldered at 48 to one of the copper strips 37 thus effecting a series connection between the coil lead 47 and the winding of the coil 49. The location of the soldered connection 48 is also shown in FIG. 9.

FIG. 8 shows a section through a ballast which is thermally protected by a fused coil 10. The ballast .52 comprises a casing 53 in which is disposed a core and coil unit 54 and a condenser 55. The elements 54 and 55 are embedded in the casing by a potting compound 56 which may be either of th asphaltic type or the resinous type, or any other suitable insulating or dielectric material. The core and coil unit 54 includes a fused coil, such as the coil 10, or 10 or 10", or 49, as above described.

In all of the constructions shown, the fusible, foil strip is embedded within the coil so thatat least the central portion thereof is not in direct contact with the potting compound 56. i

In assembling the ballast, th potting compound is introduced into the casing at two points, at eith end of the coil, and at a temperature of approximately 205 C. By the time the potting material has flowed in the lateral direction into the space between the coil and the casing, it is not hot enough to affect thefusible foil strip. This may also be partially due to the fact that the wrapper 15, or 'cover sheet 31, overlies the central portion of the fuse strip and provides a certain amount of thermal insulation for the duration of the cooling period. In the FI G.,,7 construction there is disposed on either side of the fusible foil strip 36 so much metal which acts as a heat sink, that very much higher potting temperatures could be used without affecting the embedded fusible foiLstrip.

The fused coil 10 of FIG. 8 can constitute either the primary or secondary winding of the ballast, or both, although it is preferred to fuse the primary winding.

Although only preferred embodiments of the present invention hav been described herein, it will be understood that various modifications and changes may be made in the construction shown without departing from the scope of the invention as pointed out in the appended claims.

I claim:

1. A fused coil comprising a winding having two ends, a start end, a finish end, and a coil lead, fuse means having one part electrically connected to one of said winding ends and having another part connected to said coil lead, said fuse means being disposed entirely within the confines of and in thermally conductive relationship to said coil and comprising a strip of metal foil of which the metal is a fusible alloy, said strip of metal foil being in series electrical circuit with one of said winding ends and said coil lead. v I

2. A fused coil as claimed in claim 1 in which said coil includes an overlapped wrapper, said fuse means being disposed between the overlapping portions of said wrapper.

3. A fused coil as claimed in claim 2 in which said wrapper has a window overlying said one part of said fuse means, said one winding end overlying said wrapper, and the electrical connection between said one part and said one winding end being located in said window.

4. A fused coil as claimed in claim 1 in which said fuse means is located on the Wrapper thereof, and a cover sheet overlying said fuse means.

5. A fused coil as claimed in claim 1 in which said fus means includes a copper strip electrically connected to each end of said fusible foil strip, each copper strip constituting one of said parts adapted for electrical connection.

6. A fused coil as claimed in claim 1 in. which said coil includes a winding tube, said fuse means overlying. said winding tube and beinglocated between said winding tube and said winding.

7. A fused coil as claimed in claim 1 inwhich said fusible foil strip is substantially .003 inch thick.-

8. A thermally protected ballast comprising a casing, a core and coil unit disposed in said casing, a potting compound filling the space between saidcore and. coil unit and said spacing, said core and coil unit including a winding, and a fusible foil strip in series electrical con- 5 nection with said winding, said fusible foil strip being embedded in said core and coil unit, and disposed in thermally conductive relationship to said winding.

References Cited UNITED STATES PATENTS 2,704,341 3/1955 Stacy et a1. 317-12 2,956,210 10/1960 Yatsushiro 317-40 3,173,059 3/1965 Stake 317-40 6 3,178,622 4/1965 Paul et a1. 317-12 3,225,276 12/ 1965 Daniels 317-12 3,351,815 11/1967 Wolfert 317-40 FOREIGN PATENTS 862,602 11/1962 Canada.

LEE T. HIX, Primary Examiner.

R. V. LUPO, Assistant Examiner. 

